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Environmental Factors That Influence Herbicide Performance

Environmental Factors That Influence Herbicide Performance. Dr. Eric P. Prostko Extension Weed Specialist University of Georgia Tifton. For a herbicide to work it must…. come in contact with a plant surface (root, shoot, leaves)

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Environmental Factors That Influence Herbicide Performance

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  1. Environmental Factors That Influence Herbicide Performance Dr. Eric P. Prostko Extension Weed Specialist University of Georgia Tifton

  2. For a herbicide to work it must….. • come in contact with a plant surface (root, shoot, leaves) • remain at site long enough to penetrate or be absorbed • move to its site of action

  3. Fate of Herbicides in the Environment

  4. Specific Environmental Factors • soil * organic matter, texture, CEC, pH • climatic * temperature, moisture, humidity, light intensity, dew(?)

  5. Soil FactorsOrganic Matter and Texture • most important for soil applied herbicides • Indirectly influences all processes that affect herbicides!! • the greater the organic matter and clay content, the greater adsorption of herbicides

  6. 5 4.5 4 3.5 3 Rate (kg/ha) 2.5 2 1.5 1 0.5 0 0.8 1.9 3.9 6.4 11 18 OM (%) Amount of atrazine required to reduce the growth of giant foxtail by 50% at varying OM levels. Parochetti 1973

  7. OM and texture are used to determine application rates of soil-applied herbicides.

  8. Soil FactorsCation Exchange Capacity (CEC) • soils ability to adsorb positively charged compounds • fine-textured, high-organic matter soils have larger CEC’s than coarse, low-organic matter soils paraquat

  9. Cation Exchange Capacities of OM and Clay Bailey and White 1964

  10. Soil FactorsCation Exchange Capacity • influences rate of application • not found on many herbicide labels

  11. Soil FactorspH • influences water solubility, adsorption, and hydrolysis • triazines • sulfonylureas • imidazolinones

  12. 14 12 10 8 Kd Value 6 4 2 0 3.9 4.7 5.3 6 8 pH Effect of pH on Adsorption of Atrazine McGlamery and Slife, 1966

  13. The Influence of pH on the Solubility of Accent

  14. Soil pH may influence rotational restrictions because of its effects on adsorption, solubility, and degradation.

  15. Climatic EffectsTemperature • In showy crotalaria, Blazer absorption was 4X greater at 810 and 950 than at 640.(Wills and McWhorter, 1981) • In johnsongrass, Roundup absorption doubled as temperature was increased from 750 to 950.(McWhorter et al. 1980)

  16. Climatic EffectsMoisture - PPI/PRE • activation * movement * 0.5” within 7-10 d • adsorption * availability

  17. 0 0.1" 0.25" 0.5" 1" 100 80 60 Foxtail Control (%) 40 20 0 Dual Frontier Herbicide Effect of Rainfall Amount After Application on Herbicide Performance Simmons et al. 1997

  18. Climatic EffectsMoisture - POST Dry weather causes….. • plants to develop thicker cuticles • reductions in absorption, retention, and translocation • altered metabolism

  19. Climatic EffectsRelative Humidity • Liberty treatment at 95% RH resulted in complete plant death in contrast to only a 30% inhibition in growth at 40% RH(Anderson et al. 1993)

  20. Climatic EffectsRelative Humidity • A higher relative humidity level ….. * extends drying period of herbicide droplets. * hydrates plant cuticles.

  21. Climatic EffectsLight Intensity • influences photosynthesis, cuticle development, stomatal openings, and photodecomposition

  22. Climatic EffectsWhat about dew? • studies have shown that dew can either increase or decrease foliar herbicide efficacy (Caseley 1989) • decrease - runoff and dilution • increase - cuticle hydration and uptake

  23. 0 50% 100% 100 80 60 Control - % 40 20 0 16 32 48 Spray Volume (GPA) Effect of Volume and Dew on Roundup Efficacy on Oats Dew Level Kogan and Zuniga, 2001

  24. Summary • Environmental factors cannot be controlled (except irrigation). • Need to understand environmental effects and make applications when conditions are favorable for optimum performance if possible.

  25. University of Georgia Extension Weed Science (gaweed.com)

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